Cycles render layers - make overlapping parts transparent
If your two layers are House and Grass:

House will look like this:

This says that all the objects in the scene are on layers 1 and 2, the house is on layer 1, and it should be covered up by layer 2 (the grass).
Grass will look like this:

This says that all the objects in the scene are on layers 1 and 2, and the grass is on layer 2.